Mammals and birds regulate their body temperature and generate their own body heat. Endothermic, or warm-blooded, is the result of this process, and may explain at least some of the dominance of mammals in almost all ecosystems around the world.
But until now it has not been known for sure when this endothermic appeared. Our new study, only published in Nature change that. A combination of scientist intuition, fossil discoveries from the Karoo region of South Africa, and cutting-edge technology has provided the answer: endothermy developed in the ancestors of mammals about 233 million years ago during last triassic period
The origin of mammalian endothermy has been one of the great unsolved mysteries of paleontology. Many different approaches have been used to try to determine the answer but they often give unclear or conflicting results. We think our method shows real promise as it has been validated by the presence of a large number of modern species. This suggests that endothermy evolved according to changes in mammalian bodies.
Warm-bloodedness was the key to what made mammals what they are today. Endotherms are likely the starting point from which mammals evolved: when they acquired insulating fur coats; the evolution of a larger brain, supplied with warmer blood; faster reproduction rate; and a more active life is a hallmark of mammals that evolved because they were warm-blooded.
Until recently, most scientists speculated that the transition to being warm-blooded was gradual and slow process over tens of millions of years beginning in the Permo-Triassic era, although some suggest it occurred when mammals appeared i.e., about 200 million years ago.
Instead, our results suggest that they appeared when the common ancestor of mammals lived about 33 million years ago. This new date is consistent with the existencelatest findings which is usually associated with many traits of “mammals”, such as having whiskers and fur, also evolved earlier than previously thought. And according to these results, endothermic geologically develops very quickly, which is less than one million years. Our findings suggest that the process may have been triggered by metabolic pathways and feathering.
Scientist’s intuition
Our research began with Dr Araújo and Dr David’s intuition about the inner ear. It is more than just the organ of hearing: it also houses the organ of balance that sits in the semicircular canal.
The three canals of the inner ear are oriented in three spatial dimensions. They are filled with fluid that flows in the canals as the head moves and activates receptors to alert the brain to the right of the head and body. The viscosity, or thickness, of this fluid (called endolymph) is critical to giving the organs the ability to efficiently detect head rotation and aid balance.
Julien Benoit
In the same way that a piece of butter changes from solid to liquid in a warm saucepan, or honey becomes thicker when cold, the viscosity of endolymph changes with body temperature. That means the viscosity of the endolymph will normally be altered by the evolution of higher body temperatures. But the body has to adapt because the change in viscosity will prevent the semicircular channel from working properly. In mammals, the canals adapt to higher body temperatures by changing their geometry.
The researchers realized that the changes in the shape of this semicircular canal would be easy to track through by examining fossils. Determining the species in which the geometry changes occurred, they argue, would provide accurate guidance on when endotherms evolved.
They needed fossils to test their hypothesis – and that’s where South Africa’s fossil wealth comes from Karoo region be important.
Reconstruction and research process
The arid Karoo region holds a treasure trove of fossils, many of which belong to mammalian ancestors. These fossils offer an unbroken record of the evolution of life over nearly 100 million years. They document the transformation of reptile-like animals, such as (therapsids) into mammals in great detail.
Using state-of-the-art CT scanning techniques and 3D modeling, we were able to reconstruct the inner ears of dozens of mammal ancestors from the Karoo of South Africa and elsewhere in the world. From there we were able to pinpoint which species had an inner ear anatomy consistent with warmer body temperatures, and which did not.
Read more:
What fossils reveal about the hairy history of mammals’ ancestors
One thing we should consider is the geographic position of the Karoo at the time these animals lived. It was located closer to the South Pole than it is today as a result of continental drift. That means the warmer body temperature suggested by the inner ear geometry cannot be due to a warmer climate overall. Because South Africa’s climate is on average cooler, changes in the viscosity of the inner ear fluid can only be due to the generally warmer body temperature of our mammalian ancestors.
Important time
This is an important time for our field. Until recently, to reconstruct endothermic evolution, scientists had only access to skeletal features that were questionably correlated with warm-bloodedness. Every effort is an attempt to get accurate results. The inner ear, as this study shows, changes that. We believe this may be the key to unlocking more knowledge about the ancestors of mammals in the future.
Arina Apsarini from Binus University translated this article from English.
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